Production of Reactive Oxygen Species in the Rhizosphere of a Spartina-Dominated Salt Marsh Systems

Abstract

This paper reports the presence of a metastable mixture of Fe(II), O2, superoxide and hydrogen peroxide in sediment pore water in organic carbon-rich sediments in Spartina alterniflora-dominated salt marsh systems. Field measurements at two different estuarine sites in South Carolina (one heavily urbanized and a protected research reserve) showed a broad region of reactive oxygen species (ROS) production more than 15 cm below the sediment surface within and immediately adjacent to the rhizospheres of S. alterniflora. Dissolved Fe(II) was positively correlated with hydrogen peroxide indicating a possible abiotic pathway to ROS production (r 2 = 0.94). The null hypothesis was tested that Fe(II) inventories were maintained by protective ligands and thus unreactive with respect to O2 consumption and ROS production. The addition of an Fe-binding ligand, DTPA, resulted in rapid decline of ROS in pore water, indicating that Fe(II) was labile. The half-life of superoxide under the measured solution conditions was calculated and found to be less than a second. The combination of high lability and persistent ROS was interpreted to indicate a high rate of Fe(II) and O2 supply to the pore water. The 224Ra/228Th disequilibrium was measured to determine the potential for advective mass transfer of dissolved oxygen via pore water exchange. The estimated pore water exchange of 54 L m−2 day−1 was significant but could not support the measured production of ROS alone, the direct exchange of O2 from the S. alterniflora root system may have contributed significantly to ROS production in the sediments.